In order to warn a driver that the automobile is approaching an object in its vicinity, providing a millimeter wave radar apparatus for measuring distance on each part of an automobile, for example, behind the radiator grill, the side moldings, the back panels, and the like, is under study. However, in the case in which the radiator grill and the like are provided with a sheen by using a metallic coating, this metallic coating blocks or greatly attenuates the millimeter waves. Thus, the path of the millimeter waves of the radar apparatus must be covered by a radar apparatus cover that has a sheen and has millimeter wave transparency. In order for the metallic coating to be transparent to millimeter waves, a discontinuous structure is necessary. In this discontinuous structure, the metallic coating does not form one continuous surface, but instead it has a structure (sea-island structure) in which many fine metal particles are spread over the surface in a state in which they are slightly separated from each other or have portions thereof in contact so as to coalesce into islands.
The following methods are conventionally known for forming metallic coatings having a sheen and a discontinuous structure by using a physical vapor deposition method. The industrial applicability of these methods has progressed with the goal of attaining, as coating characteristics, the effects of an improvement in corrosion resistance, conforming to stresses, and transparency to high frequency electromagnetic waves.
(1) In many conventional methods, in order to cause the metallic coating to become discontinuous to a degree that does not lose the sheen, a physical vapor deposition method is used to form a metallic coating that has a sheen and a continuous structure, and subsequently, a post-processing treatment, for example, etching using masks, is applied. However, it cannot be said that this method is optimal in terms of the treatment costs.
(2) In addition, a method has been reported in which the treatment is stopped in a state of an ultra-thin film before metallic coating formation by a physical vapor deposition method to obtain a discontinuous structure. However, in this method, a metallic sheen that is adequate in terms of external appearance cannot be obtained.
(3) As a method for solving these problems, the formation of an In or Sn coating by using a physical vapor deposition method has been reported in the following patent documents: Japanese Examined Utility Model No. JP-U-S63-30136, Japanese Patent Application Publication No. JP-A-H7-316782, and Japanese Translation of the PCT International Application No.2001-526983. In these methods, a post-processing treatment is not necessary, and it is possible to form a discontinuous structure in a state that preserves a metallic sheen. However, when taking into account coating-thickness dependence, a sufficiently stable discontinuous structure cannot be maintained.
(4) In addition, in Japanese Patent Application Publication No. JP-A-H9-70920, the present applicants have proposed a resin product made such that a base coating layer of an acrylic urethane is formed on a resin base material, a Cr thin film layer having a thickness of 150 to 800 Å is formed by a vacuum vapor deposition method on the base coating layer, and the Cr thin film layer acquires crystal grain boundaries thereby. However, although a discontinuous structure could be formed that conforms to the deformations of the base material without fracturing, a more highly resistant discontinuous coating could not be obtained. In addition, the scope of the manufacturing conditions was limited, productivity was poor, and the control of the conditions was complicated.